The invention relates to a device capable of inter-connecting two sets of optical fibers and primarily relates to the field of telecommunications equipment.
It may be used in the nodes of various types of optical telecommunications networks, especially in cross-connection or switching equipment in networks using wavelength division multiplexing (WDM) technology.
Optical cross-connection equipment is starting to be used experimentally in optical transport networks and will have to be deployed on a large scale in future years.
The reconfigurable optical cross-connection function makes it possible, in a node of the WDM network, to establish and reconfigure connections between the incoming optical channels and the outgoing optical channels.
It primarily meets the needs for securing the network in the event of link or node breakdowns, by virtue of the possibility of bypassing nodes by the WDM channels.
In this case, the optical cross-connects are reconfigured by network administration bodies. This possibility of reconfiguring optical cross-connects more generally makes it possible to change from a juxtaposition of point-to-point WDM links to a genuine flexible optical layer whose granularity is the wavelength (or optical channel), that is to say 2.5 or 10 Gbit/s. This need for flexibility of the optical layer is in particular linked to the increase of Internet traffic and to the need to manage the increasingly large WDM transmission capacities which result therefrom.
More generally, this invention relates to all the fields where optical beam routing is necessary.
Switching matrices for optical cross-connects have been produced using electronic technology. Several manufacturers (Tellium, Nortel, Ciena, Monterey Networks, Sycamore, Nexabit Networks) have announced products using this technology.
The all-optical cross-connection technologies are less advanced. They are transparent at the bit rate of each optical channel, and will therefore allow better upgradeability of equipment in a multiple seller and multiple bit rate environment. On the other hand, it is relatively clear that these technologies can only be imposed on the market if their cost is competitive with respect to the electronic technologies.
Among the all-optical technologies available, solutions called optomechanical solutions have good maturity and excellent optical performance (DICON, JDS, AMP, etc.). Nevertheless, they are characterized by a large overall size and a price which quickly becomes prohibitive depending on the number of ports.
Integrated thermooptical matrices are also available, either made using polymer technology (JDS), or silica technology (NEL). Obtaining a number of ports greater than 16 remains a problem.
Other integrated technologies, such as lithium niobate or indium phosphide still require significant developments in order to arrive at high-performance and high-capacity matrices.
The above technologies suffer from a lack of integration (in the case of Nxc3x97N systems, with N inputs and N outputs, based on the use of 2N discrete deflectors 1xc3x97N) or require a high number of elementary components limiting the possibilities of integration counting from N=16 or 32 (case of planar integrated matrices).
Also, the research effort is oriented toward solutions capable of offering, a few years hence, high capacities at a realistic cost, based on technologies having already acquired a certain maturity away from the field of conventional optoelectronics.
Microelectromechanical systems (MEMS) on silicon are being studied for optical cross-connection applications, mainly in the USA (ATandT, IMMI, OMM, Astarte, Lucent, Xros, etc.).
These systems use switching matrices based on micromirrors on silicon capable of deflecting an optical beam along two axes. An optical cross-connect having 576 ports has been produced by Texas Instruments and Astarte. On its part, Lucent announces the marketing at the end of 2000 of a xe2x80x98Wavestar lambda routerxe2x80x99, having 256 ports. For its part, Xros presents a prototype having 1152 ports, with provision for marketing at the start of 2001.
The use of micromirrors is especially beneficial from the point of view of wavelength insensitivity and polarization independence.
However, this emerging technology still raises questions with regard to reliability, angular control and manufacturing efficiency, in the case of matrices having several tens of micromirrors, each one having a diameter of a few hundred xcexcm.
Liquid crystal technologies, which have a good level of maturity for display applications, also provide interesting perspectives. NTT and France Txc3xa9lxc3xa9com have produced various demonstrations, by cascading several stages of liquid crystal cells and of birefringent calcite crystals (for example 11 stages for 64 ports).
Smaller-capacity devices, also using the rotation of polarization in a liquid crystal, are proposed in Japan by NEL, and in the USA by Chorum and Spectraswitch.
The use of diffraction gratings created in high-resolution liquid crystal cells has also been envisioned for several years (NTT, University of Cambridge, ENST Bretagne, France Txc3xa9lxc3xa9com). In order for two sets of monomode fibers to be interconnected efficiently, it is necessary to cascade two deflection stages. This approach has been used in a 16xc3x9716 demonstrator based on two linear arrays of holographic gratings recorded on a photosensitive support [6] and also in an 8xc3x978 system using two linear arrays of liquid crystal deflectors.
In addition, the general use of diffraction gratings based on electrically addressed liquid crystals has been proposed by various laboratories, in diverse applications.
In these devices, an electrical voltage applied locally to the terminals of a liquid crystal of suitable type makes it possible to create a local variation in refractive index or birefringence.
By making this value vary along one or two axes it is possible to create a structure which will diffract an incident beam in one or more preferred directions, depending on the spatial profile of the index variation: thus a beam deflector functionality is obtained.
To date, work in the field of optical routers is limited to devices in which the beams undergo a single deflection (cf. work by NTT [1], of the University of Cambridge [2, 3], and by ENST Bretagne [4]). This approach is suitable for matrices of 1xc3x97N type with 1 input and N outputs [1, 3, 4] or possibly for Nxc3x97N matrices of low capacity [2]. This is because, in this last case, a loss factor 1/N has to be introduced with monomode fibers. The capacities demonstrated experimentally with this approach remain modest: 1xc3x978 [5] and 1xc3x9714.
The liquid crystal devices proposed only allow a small number of fibers to be connected.
Liquid crystal router devices deflecting the optical beams in two perpendicular dimensions have also been proposed. However, the proposed devices prove to be bulky since the deflection means have to be powerful.
The invention mainly proposes to solve this drawback, that is to say to provide an optical router in two dimensions in which the deflection means are less bulky, while minimizing the optical losses and while adopting a spatial frequency band of reasonable spread.
The invention proposes to solve this drawback by virtue of an optical beam routers comprising a series of optical input channels and a series of optical output channels, two optical spatial index modulation cells capable of deflecting an optical beam coming out of an input channel and arriving onto an output channel, respectively, characterized in that each series of optical channels is distributed in two dimensions transverse to the direction of the channels and in that the spatial index modulation cells are each provided in order to produce deflections in these two dimensions.